In the prior art, in image forming apparatuses such as a copier, a scanner, and a multi-function peripheral including functions of the copier and scanner, etc., an ADF (Auto Document Feeder) for feeding documents from a feeding tray to an output tray via a feed path has been known. Among such ADF's, one configured to execute a switchback feed of the document so that, for example, text printed on both sides of each sheet of the document has been known. In the bidirectional feeder, the leading end and the trailing end of a sheet are switched and both sides of the sheet are scanned consecutively. An example of such a document feeder is disclosed in Japanese Patent Provisional Publication No. HEI 8-85649 (hereinafter, referred to as '649 publication).
FIG. 17 shows a feeding path of a document feeder suited for the double-sided reading of documents. As shown in FIG. 17, document P is placed in an input tray 100 with the first sheet being the topmost sheet of the document P. The uppermost sheet is fed toward a feeding path 102 as a pickup roller 101 rotates. In the feeding path 102, the document P is fed by feed rollers 103 provided at various positions of the feeding path 200. When each sheet passes through a reading position X (indicated by arrow), a first surface of the sheet P is read using an image reading device such as a CCD or CIS. When a sensor detects the trailing end of the sheet P of which the first surface has been read, discharge rollers 104 are stopped with nipping a trailing end portion of the sheet P so that the sheet P is also stopped.
Thereafter, as the discharge rollers 104 reversely rotate, as shown in FIG. 18, the sheet P is fed into a bidirectional feed path 105. Then, the sheet P re-enters, from the bidirectional feed path 105, the feeding path 102 at an upstream side portion of the reading position X. With this movement, the leading end and the trailing end of the sheet P are exchanged. As the sheet P is fed by the feed rollers 103 and passes the reading position X, the second surface of the sheet P is read by the image reading unit. Then, when the sensor detects the trailing end of the sheet P after the second surface was read, the discharging roller 104 are stopped again with nipping the trailing end portion of the sheet P. Thereafter, the sheet is reversely fed in the bidirectional feed path 105. When the sheet P is re-enters the feeding path 102 from the bidirectional feed path 105, the leading end and the trailing end are exchanged again, i.e., the sheet P is oriented such that the first surface faces the reading position X. Then, the sheet P is fed in the feeding path 102 and discharged in an output tray 106 with the first surface being oriented downward (i.e., facedown discharge). With the above movement, both the first and second surfaces are read, and the sheets P are stacked on the output tray 106 in the order similar to the order the sheets P were stacked on the input tray 100.
The pickup roller 101, feed rollers 103 and discharge roller 104 are rotated as driving force is transmitted from a motor. Specifically, the pickup roller 101 and the feed rollers 103 are always rotated in a predetermined direction for feeding the sheet P forwardly (i.e., from the upstream side to the downstream side of the sheet feed path 102). The discharge rollers 104 are rotated either forwardly or reversely for bidirectional feed. For example, as shown in FIG. 18, when the sheet P is nipped by the feed rollers 103 and the discharge rollers 104, feeding directions of the feed rollers 103 and the discharge rollers 104 should coincide with each other. If the sheet P is nipped by the feed rollers 103 on immediately upstream side of the reading position X and the discharge rollers 104, the feeding direction of the feed rollers 103 on immediately upstream side of the reading position X and the discharge rollers 104 should coincide with each other.